1. Field of the Invention
The present invention relates to an image forming apparatus, which forms a toner image on an image bearing member, and transfers the toner image on a transfer material, thereby forming an image on the transfer material.
2. Related Background Art
In general, as a four-color full color image forming apparatus, there has been known an image forming apparatus comprising four pieces (four-colors) of image forming parts forming a different color toner image, respectively. Four pieces of image forming parts are lined up and disposed from an upper stream side to a down stream side along the moving direction of an intermediate transfer belt, and each color toner image formed by each image forming part is primary-transferred in order on an intermediate transfer belt, and the four-color toner images are superposed on the intermediate transfer belt. After that, these four-color toner images on the intermediate transfer belt are secondary-transferred collectively on the transfer material, and these four-color toner images are heated by a fixing apparatus, thereby fixing a four-color full color toner image on the transfer material.
Here, an applied starting timing and an applied stopping timing of a developing bias voltage and a charging bias voltage at the recording operation (image forming operation) starting time and the recording operation (image forming operation) completing time will be described.
In each image forming part, at the recording operation starting time, the photosensitive drum, when coming to stable rotation, drives a charge device to start charging. When an area where the photosensitive drum surface and the developer held in the developing sleeve of the developing device are brought into contact is taken as a developing area, the photosensitive drum surface coming close to the developer sleeve is 0 [V] until a charged area (hereinafter referred to as “a charging portion”) within the photosensitive drum surface reaches a developing area N. Consequently, in case the developing bias voltage (for example, −550 [V]) of the developing sleeve is turned on until the charging portion on the photosensitive drum reaches the developing area, a large contrast potential Vcont (=|0−(−550)|=550 [V]) is formed in the whole photosensitive drum surface area which passes through the developing area, a toner within the developer ends up adhering to a non-image part before the developing starting time. As a result, a toner stripes image is formed before the image part in the transfer material P.
On the other hand, to prevent the toner from adhering to the non image part, the developing bias voltage of the developer sleeve may be turned on after the top end of the charging portion of the photosensitive drum surface charged to a surface potential Vd (for example, −700 [V]) completely passes through the developing area. However, under such situation, before the developing bias voltage is turned on (when the developer sleeve is 0 [V]), since a portion of the photosensitive drum surface charged to Vd (=−700 [V]) passes through the developing area, at this time, between the developing sleeve and the photosensitive drum surface, there is formed a large fog-taking potential Vback (=|−700−0|=700 [V]) is formed, and in case a two component developer comprising a toner and a carrier is used as the developer, there arises a problem that the carrier in the developer adheres to the photosensitive drum surface.
To prevent not only the toner but also the carrier from adhering, though the developing bias voltage may be turned on simultaneously in timing of the charging portion on the photosensitive drum reaching the developing area, in reality when considering various types of deviations, it is difficult to allow on-timing to be completely matched every time. Further, since the developing area has some constant width, even from that view point, it is difficult to allow the timing to be matched. As a result, either the carrier or the toner on the developing sleeve ends up adhering to the photosensitive drum. Particularly, when the carrier of the developing sleeve flies to the photosensitive drum, the carrier harms the photosensitive drum surface by a cleaning blade of a cleaning device to clean the photosensitive drum surface, thereby shortening the life of the photosensitive drum. Hence, heretofore, by turning on the bias voltage of the developing sleeve immediately before the top end of the photosensitive drum surface charged to Vd (=−700 [V]) by the charging device reaches the developing area, the adherence of the carrier is completely prevented. As a result, a toner stripes image has been formed before the image part. The toner is different from the carrier, and is hard to harm the photosensitive drum by the cleaning blade. When configured in such a manner, though there arises a problem that the amount of consumption of the toner increases, there is no increase in the adherence of the carrier with the photosensitive drum, and it has been possible to maintain an excellent image for a long period of time (Japanese Patent Application Laid-Open No. 2003-280483).
On the other hand, at the recording operation completing time also, since a problem similarly to the recording operation starting time arises, the rear end of the charging portion of the photosensitive drum surface charged to Vd (=−700 [V]) by the charging device has turned off the developing bias voltage toward the developing sleeve immediately after having passed the developing area.
As described above, as the application starting timing and the application stopping timing of the developing bias voltage and the charging bias voltage at the recording operation staring time and the recording operation completing time in which the above described configuration is used, instead of the carrier not adhering to the photosensitive drum, a toner image with toner stripes (toner stripes image) is formed before and behind the image part.
This toner stripes image is once transferred on the intermediate transfer belt by a primary transfer roller, and after that, it is conveyed up to a secondary transfer roller. Here, though the toner image of the image part is transferred on the transfer material, the toner stripes image before and behind the image part is not transferred on the transfer material, but on the secondary transfer roller, and smears the secondary transfer roller. To remove such smearing, it has been quite usual for a secondary transfer roller cleaner to be provided.
Now, in the four-color full color image forming apparatus, if the timing of the application and the stopping is set up as described above, the toner stripes image is formed before and behind the image part for each color. These toner stripes images are transferred together on the intermediate transfer belt 7 when the toner image of each color is primary-transferred on the intermediate transfer belt by electrical operation and pressing force of the primary transfer roller. At this time, while four-color toner images are superposed on the intermediate transfer belt, similarly, the toner stripes images are also superposed for four-colors on the intermediate transfer belt. As a result, these toner stripes images are conveyed up to a secondary transfer portion with four-colors superposed in the four-color full color image forming apparatus.
And yet, this toner stripes image, as described above, is subjected to a developing step with an extremely large contrast potential Vcont (=550 [V]) since the developing sleeve is applied with the developing bias voltage Vd (=−550 [V]) when the surface potential of the photosensitive drum is 0 [V]. Usually, the surface potential V1 of the photosensitive drum of a solid image part often uses a value smaller than 0 [V] in view of stability (for example, −200 [V]), and as a result, even when comparing with the contrast potential Vcont (=|V1−Vdc|=|−200+550|=350 [V]) at the usual solid image forming time, the toner stripes image before and behind the image part has an extremely large amount of toner loaded. That is, before and behind the four-color full color toner image on the intermediate transfer belt, there is formed a toner stripes image superposed with the toner more than equivalent to the solid images for four-colors.
After that, following the rotation of the intermediate transfer belt, the four-color toner image and the toner stripes image are conveyed up to the secondary transfer portion, and from among them, while the four-color toner image is transferred on the transfer material, the toner stripes image formed before and behind the image part does not contact the transfer material, but directly contacts a secondary transfer roller 9, and ends up being transferred on the secondary transfer roller by electrical operation and pressing force of the secondary transfer roller. As a result, the secondary transfer roller is smeared.
Although such smearing can be removed by the above-described secondary transfer roller cleaner, this cleaner sometimes is unable to remove the smear by one time cleaning only for the toner stripes image superposed with four-colors. When the rear surface of the transfer material contacts the smeared portion in the next step, there arises a problem that the rear surface of the transfer material ends up being smeared. Particularly, depending on the material of the secondary transfer roller, there are the cases where the cleaning removal is difficult. For example, in case the surface shape of the secondary transfer roller is in a dilapidated state, it is difficult to be removed, and the above described problem is prone to occur.
In contrast to this, there is a method available, in which the secondary transfer roller is allowed to rotate for several turns in a state in which no image is formed, and after the cleaning of the secondary transfer roller is completely performed, the supply of the transfer material is made so as to restart the image formation. However, since the image formation is not possible to be performed during such cleaning of the secondary transfer roller, if sufficient cleaning of the secondary transfer roller is awaited, the lowering of efficiency is invited.
Further, when the toner stripes images for four-colors are conveyed to the secondary transfer portion, the secondary transfer roller is applied with a second transfer bias voltage of the polarity in reverse to the usual polarity, so that the smearing of the secondary transfer roller can be prevented. However, at this time, the similar problem arises in the intermediate transfer belt cleaning material provided on the intermediate transfer belt of the down stream of the secondary transfer portion, and the smearing of the rear surface of the transfer material or the lowering of the efficiency occurs.